1. Field of the Invention
The present invention relates to Global Positioning System and more particularly, to an apparatus and a method for synchronizing the time on peripheral nodes that are not equipped with GPS receivers by using Global Positioning System (GPS) information of a node equipped with a GPS receiver in a communication system.
2. Description of the Related Art
In general, GPS service refers to a service wherein upon receiving GPS signals transmitted from at least three GPS satellites, accurate time and distance are determined from the received GPS signals. That is, with three distances different from one another measured by a triangulation method, a user is provided information regarding their current position. In addition, by further using data, such as a map or a traffic report, etc., the above GPS is frequently used as a navigation service in which when a user selects a specific destination, the combination of navigation information, such as transportation information or information on guidance of the roads, etc., and the position of the user may be used to provide appropriate turn-by-turn directions to the selected destination Also, the above GPS is being applied to the field of geodetic survey and surveying, military purpose, aeronautical navigation control, and the like.
In a wireless communication network, synchronization of a system or a network is an essential element. At present, as a method for synchronizing a wireless communication network, a synchronization method using a OPS satellite corresponds to a representative method, and in order to receive a GPS signal, point-to-point topology between a receiver and the GPS satellite is employed.
FIG. 1 is a configuration block diagram illustrating a general GPS receiver. The GPS receiver uses information of GPS satellites with a view to time synchronization uses, as reference, an input of 8 kHz signal synchronized with a GPS signal or GPS one (1) Pulse Per Second (1 PPS), and performs the function for providing a system with 10 MHz signal, a Pulse Per 2 Second (PP2S) signal, and an 1 PPS signal.
To examine the respective configurations in more detail with reference to FIG. 1, the GPS receiver 10 includes an antenna interface 110, Field-Programmable Gate Array (FPGA) 120, a GPS receiver 130, a Central Processing Unit (CPU) 140, an oscillator 150, and an input/output unit 160.
The antenna interface 110 performs the function of providing a 1 PPS signal synchronized with Universal Coordinated Time (UCT) on receiving a signal L1 from a GPS receiving antenna, and the function for checking a physical connection state with the GPS receiving antenna to report the checked physical connection state to a system.
The FPGA 120 includes an alarm detector 121, a multiplexer 125, a phase error detector 122, a discrete input/output interface 123, and a CLocK (CLK) and timing generator 124. Herein, the alarm detector 121 performs the function for determining if an output of each Voltage Controlled Oscillator (VCO) in the GPS receiver 10 exists, if power is normally supplied in the GPS receiver 10, etc., and reporting the determination to the CPU 140. The multiplexer 125 receives 8 kHz signal synchronized with the GPS 1 PPS signal and an external 1 PPS signal. The phase error detector 122 receives an output of the multiplexer 125 selected according to a selection control signal, and inspects a phase error of the received output signal of the multiplexer 125. The discrete input/output interface 123 performs an input/output operation of error signals among the CLK and timing generator 124, the phase error detector 122, and the alarm detector 121. The CLK and timing generator 124 generates an 1 PPS and PP2S output signals by using a GPS signal or a synchronized 10 MHz clock.
The GPS receiver 130 processes the GPS signal received by the antenna interface 110, and provides the GPS 1 PPS signal to the FPGA 120.
The CPU 140 controls each of the configuration elements of the GPS receiver 10 during a GPS receiving operation, and reports a current reception state of the GPS receiver 10 to the system on sensing an alarm reported by the alarm detector 121. The above reception state can be set to either a Function Failure (FF) state, a Power Failure (PF) state, a normal state, an abnormal state, or a holdover state.
The oscillator 150 is constructed by an Oven Controlled X-tal (crystal) Oscillator (OCXO) or a Temperature-Compensated X-tal (crystal) Oscillator (TCXO), and provides an output signal having an oscillation frequency that is mechanically and physically stable. The OCXO uses characteristics with which a crystal susceptibly changes in response to temperature, and employs a method in which the temperature around the crystal is constantly maintained by using an oven so that no error can occur. Even if the OCXO has better accuracy than any other product to which the crystal is applied, it is bulky and uses various sources of electricity, including 12, 24, and 30 Volts. It is mainly used for a repeater or military use, such as in a missile, a satellite, and the like, rather than Personal Communication Service (PCS). Since TCXOs are less expensive than the OCXOs, they are used in many general purpose GPS receivers. In order to improve the operation performance of the crystal, the TCXO employs a temperature compensation circuit, a thermistor, and a VCO. The temperature compensation circuit puts a limitation on the output frequency variation of the TCXO according to an operation temperature change. The thermistor reduces an oscillation frequency error of an oscillator fluctuation according to temperature. The VCO has high frequency stability in relation to temperature changes ranging from several MHz to dozens of MHz, and is extensively used as reference frequency sources.
By supplying a user with a Universal Asynchronous Receiver/Transmitter (UART) port including a debug port and a Time Of Day (TOD), the input/output unit 160 not only enables the user to monitor current TOD data in real-time by using the TOD port, but also can provide the function of remote control and download.
On the basis of predetermined reference, e.g., midnight, Jan. 6, 1980, the TOD begins to count from the first GPS 1 PPS, and by giving notice in what order a currently received 1 PPS is counted, an accurate time information through this count operation can be provided. Also, since the 1 PPS corresponds to an exact timing signal, respective nodes synchronize all clocks employed in a system with the 1 PPS signal.
A synchronization method of a prior wireless communication network uses a scheme in which the GPS information is received from the GPS satellites by the GPS receiver equipped as described above, and the synchronization is implemented with the received information. However, in the case of either a multistory building, an urban downtown with many obstacles, or indoor where it is difficult to receive the GPS signals, it is difficult for the above scheme to receive the GPS information from the GPS satellites, and therefore, problems can be raised in the synchronization of a system.